Because the Zeta kits are notoriously flimsy and floppy. Ask an owner of a Zeta kit, particularly one of the older ones, how satisfied they are. A model made from syrene plastic will not be anywhere as near as durable as a toy, and if you can't transform it more than a few times, what's the point? Also, what they don't show you in the model magazines is the touch-ups that these models require after some posing and transforming. I decided long ago that I had spent enough time painting kits to have the paint scraped off by transforming the model.